Electrolytic recovery, for example, is used as a hydro-metallurgic method when production of pure metal, such as copper, is the objective. In the recovery electrolysis, copper is reduced directly from the electrolytic solution, which is a copper sulphate solution. In the process, copper is precipitated on to the surface of cathodes made of, for example, acid-proof steel, whereupon the copper is removed mechanically from the plate surface. The anodes are insoluble metal plates in the process. The precipitation rate of the metal, such as copper, depends on the current density, but this can not be increased indefinitely without lowering the quality of the precipitate. In practice, the highest possible current density is determined by the so-called critical maximum current density, that is, the highest current density, when the precipitate is still of a sufficiently high quality, which is proportional, for example, to the content of metal to be precipitated and inversely proportional to the thickness of the so-called diffusion layer.
It is known in the art to boost electrolytic recovery by bubbling, that is, by blowing gas into the electrolyte basin. The mass transport on to the cathode surface improves, because bubbling reduces the thickness of the diffusion layer. It is hereby possible to use a higher current density without lowering the surface quality of the precipitate.
A method and equipment for bubbling in electrolytic recovery are known from the US 2007/0251828 publication. According to this method, the process produced air bubbles with a diameter of 0.5-3 millimeters, and a pipe system of a porous material is used for supplying bubbling gas into the basin.
It is also known that the use of very small air bubbles is more advantageous for the recovery process as it promotes the production of a thinner diffusion layer on the cathode surface, which will for its part allow the use of a higher current density without resulting in a poorer precipitate quality.